Sugarcane ethanol had the smallest "carbon debt," which takes about 17 years to pay off, Tilman said. Corn ethanol's debt would takes 93 years to pay off.

When tropical rain forests are cleared for growing palm, the crop's biodiesel carries an 86-year debt. Even worse, when peatland rain forests are cleared for the crop, they rack up a 423-year carbon debt.

Tropical Clearing

This clearing of rain forests for biofuel is happening in Brazil, Indonesia, and Malaysia, and will probably start happening in the [Democratic Republic of the] Congo," Tilman said. "Economics favors production of these biofuels, since they're cheaper than petroleum"—even though they may be worse for Earth's climate, he added.

Roel Hammerschlag, of the Stockholm Environment Institute U.S. in Somerville, Massachusetts, also expects increased use of biofuels will drive land clearing in those countries.

"A large fraction of the land appropriate for biofuels lies in developing nations who can and should benefit from the new markets," Hammerschlag said.

"I do hope that these articles will launch a bigger debate [on biofuels]," he added.

Sue Page of the University of Leicester in England is an expert on tropical peatland forests.

"There is no way to avoid the problems [of growing crops for biofuels in tropical regions]," she said.

The only way to prevent massive carbon dioxide emissions is not putting plantations on peat soils in the first place, Page said.

The maximum lifespan for oil-palm cultivation on lowland tropical peat is less than a hundred years, because the land tends to get too compact and floods, Page said.

"The carbon debt incurred in developing the peatland for biofuel production can never be repaid," she said.

Cropland Explosion

With foods crops increasingly being turned into fuels, the world will need more cropland to feed everyone, according to another study also being published in Science this week. That study focused on the effects of the explosion of corn for biofuel in the United States.

For every 5 acres (2 hectares) devoted to corn to make ethanol, the world would need more than 4 more acres (1.6 more hectares) of cropland for food. So increased use of biofuels may drive farmers to convert more native ecosystems into cropland, damaging the environment in the process, the study authors argue.

Other kinds of biofuels could still be climate-friendly, since they wouldn't cause carbon dioxide emissions from land use changes, Tilman said. These include biofuels made from switchgrass and prairie grasses, waste materials from forests, and algae grown in vats. "These all work in the lab, on a small scale," Tilman said.

"We'll see which of these work on a large scale. We're still in the learning phase."